Currently, satellite positioning systems, such as GPS, are one of the most accurate location data sources available to portable or mobile electronic devices. However, there are a number of drawbacks associated with satellite positioning systems. For example, it might not be possible to receive signals from the satellites when the device is indoors, under heavy foliage or in an ‘urban canyon’ (i.e. between a number of tall buildings), making it impossible to obtain a location measurement (sometimes referred to as a ‘fix’). Satellite positioning systems can also be prone to errors in the location measurement which can be due to a number of different reasons, including ‘multipathing’ where the signals from a satellite can reflect off of buildings before reaching the satellite positioning system receiver. These errors can cause the reported location to be some distance from the actual location, sometimes even as far as several city blocks. Another drawback with satellite positioning systems is that the receiver consumes a relatively large amount of power while making a location measurement.
Although the satellite positioning system receiver can be manually activated and deactivated by a user of the device to help reduce the power consumption, when some event occurs where it is useful to know the exact location of the device (for example if the user of the device is placing an emergency call and needs to provide their exact location, or the user of the device suffers a fall or other accident and the device is configured to automatically request assistance for the user), activating the satellite positioning system receiver and attempting a measurement is not without risk, as it might not be possible to get a measurement in the current location of the device.
Therefore, in such situations, it can be useful to make use of the last known location of the device obtained using the satellite positioning system receiver before the satellite signal was lost. To do this, the satellite positioning system receiver must either collect location measurements continuously (in which the receiver will quickly drain the battery of the device), or a ‘breadcrumbing’ technique is used, in which the satellite positioning system receiver is selectively activated by the device to intermittently take location measurements. As the receiver is not continuously powered or active, there is some reduction in the power consumption of the device. If the receiver is unable to determine the location of the device when it is activated, the last acquired location measurement (breadcrumb') can be used as an estimate of the current location of the device.
One particular breadcrumbing technique is described in US 2006/0119508. This document describes a method and apparatus for conserving power on a mobile device through motion awareness, in which measurements by an accelerometer and the GPS receiver in the mobile device are used to determine whether the mobile device is in motion. If it is determined that the mobile device is not in motion, the scanning by the GPS receiver can be halted or reduced to a lower duty cycle to conserve power in the mobile device. When it is subsequently determined from the accelerometer data that the mobile device is in motion again, the GPS receiver resumes scanning and measures the current location of the mobile device.
Although this breadcrumbing technique can reduce the power consumption of the device compared to continuous operation of the satellite positioning system receiver, it is desirable to improve the existing breadcrumbing techniques to further reduce the power consumption of a portable or mobile device.